Variable directional coupler having a movable articulated conductor



Jan. 19, 1965 J ocK ETAL 3,166,723

VARIABLE DIRECTIONAL COUPLER HAVING A MOVABLE ARTICULATED CONDUCTORFiled March 6, 1961 2 Sheets-Sheet 1 COUPLER \o 28 MOVABLL COUPU NC:SECTION COUPLED OUTPUT s \GNALS MA/ev/N J Bock F/20/?/ c/ S. HOWARDINVENTORS ZOVLJHI zztm \COUDLER O 15v W Jan. 19, 1965 M. J. BOCK ETAL3,166,723

VARIABLE DIRECTIONAL COUPLER HAVING A MOVABLE ARTICULATED CONDUCTORFiled March 6, 1961 2 Sheets-Sheet 2 MOVABLE COUPUNG ECTlON 54) ABSORBERABSORBER I MOVABLE. COUPLING 5ECT\ON as MARI 0v .x BOCK FREDER/ CK 5. HoWA RD INVENTORS A 7TORNEY$ United States Patent 3,166,723 VARIABLEDRECTIQNAL C-IB'UPLER HAVHNG A MGVABLE ARTECULATED (IUNDUCTOR Marvin J.Bock, Santa Barbara, and Frederick Howard, Los Angeles, Caiii,assignors, by mesne assignments, to Micro-Radionics, l'nc., Van Nuys,Calif.

Filed Mar. 6, 19b1, Ser. No. 93,530 3 (Ilaims. (Cl. 333-119) Thisinvention relates to devices for controlling the transmission of Wavesat microwave frequencies, and more particularly to devices for providinga mechanically variable coupling, attenuation, and amplitude control formicrowave systems.

Couplers, attenuators, modulators and the like are basic components inmicrowave systems. Often a number'of different functions may beperformed by a single device, so that a coupler, for example, might beable to serve additionally as an attenuator or modulator. This may bebetter understood by reference to the senses in which the termscoupling, attenuation and modula tion are usually employed. The termcoupling is used to indicate the transfer of a predeterminedor selectedamount of microwave energy from one conductor to another. The termattenuation is used to indicate the controlled dissipation of a selectedamount of microwave energy which is propagated along a conductor. Wherethe attenuation is varied with time in some predetermined fashion, thismay of course constitute an amplitude mod ulation. It is thereforeevident that certain kinds of couplers may also operate as attenuators,and if they operate variably with time in addition serve the function ofmodulators.

A mechanically operable coupler which can perform all these functions iswidely sought for microwave systems. Such devices have not heretoforebeen provided because of a number of ditficulties which have beenencountered in getting satisfactory electrical and mechanicalperformance. Many fixed couplers are known, but those which have wideband performance are usually not susceptible of. conversion to avariable operation. Variable couplers have usually had limited bandwidth, and particularly have been subject to troublesome nonlinearitiesin operation. The non-linearities have required complicated mechanicalmotions in order to achieve accurate coupling settings, which in turnhave greatly increased costs, limited accuracy and madereproducibilityextremely difiicult.

A microwave variable coupler of suitably small size and low cost whichcan be coupled into standard guide systems and testing equipment widelyused at microwave frequencies.

It is therefore an object of the present invention to provide animproved microwave variable coupler.

A further object of the present invention is to provide an improvedvariable couplerwhich is precisely adjustable but which'is simple andeconomical to construct.

Yet another object of the present invention is to pro- Tvide an improvedwave coupling device for microwave applications, which device may serveas a variable coupler,- attenuator or modulator. 5 -Another'o'bject ofthe present invention is to provide an improved mechanically variablewhich operates bidirectionally.

In accordance with the invention, variable wave coupling between twoconductors is provided by transverse movement of one conductor sectionrelative to another and the employment of an articulated conductiveconnection between the movable section and terminal points.

In a specific example of a mechanically variable coupler section inaccordance with the invention, the coupler is arranged as a four-portslab transmission line device. A principal center conductor section isfixedly coupled between two of the ports, while the remaining two portsare coupled by an articulated center conductor section including acentrally located coupling element. The coupling element is transverselymovable relative to the principal center conductor by a linear motionmechanism, and may occupy different positions in a wave coupling region.The relative transverse spacing between the center conductors in thecoupling region exercises an ex-' tremely precise control over theamount of coupling. Further, incident waves may be propagated on eitherthe principal center conductor or the articulated center conductor, sothat the device is fully bidirectional. The entire coupler is extremelycompact, simply constructed although linearly variable with changes ofthe control element, and fully compatible with coaxial and other guidedwave systems. With the addition of an attenuator device at a selectedterminal, waves variably coupled to the selected terminal may bedissipated so that the device functions as a variable attenuator.Further, the mechanical positioning of the coupler section may be variedwith time so as to provide amplitude modulation.

Another aspect of the present invention is the provision of means bywhich the frequency response of the coupler may be varied as desired. Insuch arrangements, the coupling lengths of the center conductors aremade not only transversely movable relative to each other, but are alsolongitudinally variable. Through the longitudinal variation, the lengthalong the center conductors by which coupling is effected may. bechanged to be optimum for any given frequency. Accordingly, the alreadyinherently wide band characteristics which the device provides may beextended to any useful rangewhich is desired.

The novel features ofthe invention maybe better understood by referenceto the following description, taken in conjunction with theaccompanying. drawings, in which: 7 f

FIG. 1 is a perspective view, partly broken away, of one form of guidedwave coupler in accordance with the present invention;

FIG. 2 is a plan view of the arrangement of FIG. 1;

FIG. 3 is an enlarged perspective view of a fragment of the arrangementof FIG. 1;

FIG. 4 is a sectional view of the fragment of FIG. .3, taken along theline 44 in FIG. 3 and looking in the directionof the appended arrows;

FIG. 5 is a plan view, partly broken away, of a wide band variablecoupler in accordance with the present invention, and

FIG. 6 is a perspective view of a fragment of the wide band variablecoupler of FIG. 5.

A bidirectional variable coupler 10 (FIGS. 1-3) according to theinvention employs slab transmission line construction for transmittingwaves at microwavefrequencies. Slab transmission line is a widely usedguided wave'deyice. Because it is essentially an open-sided coaxialline, it is often used in conjunction. with-coaxial line systems andrelated systems, such as st rip tra1'1smissionline units. A'specificexample is provided, by way 'of illustratiohfof a coupler coveringatwotoone frequencyrange and operating at a nominal frequency ofPatented Jan. 19, 1965 microwave coupler sitions and impedance changes.tral coupling element is connected both electrically and 6kilomegacycles (hereinafter kmc.) in the range 4 to 8 kmc. The main bodyof the coupler 1G is provided by a frame 12 of generally rectangularconstruction, the principal electrical elements which are used being atop plate 14 and a bottom plate 1'5, these plates serving as V theground vplane conductors for the slab transmission lines. '.The'remainder' of the frame 12 may be completed by eppropriateside and endelements which may or may not be conductors as desired, althoughconductive elements tend to reduce external radiation to a low magnitudeand are preferred.

.tially filled with dielectric material between their center'conductorsand their outer conductors.

For smooth coupling of Waves from the slab lines to the coaxial lines,and vice versa it is preferred to utilize an air transition section inthose parts of the terminal conductors 26-23 which are directly joinedto the associated center condoctor of, the slab lines. The various inputand output terminals FIG; '1 have been labeled for convenience withdifferentinput and output designations, but these areinerely'.illustrative of different wa'ys'in which conplings ina'y'be' etfe'ctedl,Inithis example, the first ter minal conductor is directly coupled to asystem load 24 through the second terminal conductor 21. The coupledoutput signals are derived at the third terminal conductor 22, and maybe applied to an associated system or matched termination (not shown)depending upon the use intended for the coupler It]. The fourth terminalconductor 23 is here unused, and is properly terrninated in a matchedtermination 25.

The coupler ltrincludes a fixed or principal length of 'slabflinece'ntei 'co'nduct'or26 coupled between the center conductors of theoppositely disposed first and second i terminal conductors 20 and'21. Amovable coupling 1 section 28 forms a slab line center conductor whichcon ne'cts the center conductors of the third and fourth terininalconductors 22 and 23. At each end of the movable section' 28, a'fixedlength of slab line center conductor 36," 31 is attached to the adjacentterminal condoctor 22, 23, respectively. In the approximate center ofthe movable coupling section 28, the slab center conductor is .i'nithe.form of. a generally U-shaped coupling element. 33 whose principallength is parallel to the fixed center" conductor '26.' The volumebetween the ground plane conductors 14-, 15 which encompasses theassociated'lengths of the fixed and movable center conductors 26, 33 maybe termed a Wave coupling region. The free. ends and the corners of theU-shaped coupling element 33 are designed in accordance with well knownslab line construction techniques to minimize sharp tran- Each end ofthe cen mechanically to the closest fixed end or 31 of the movablecoupling section by a different pair of articulated arms 35, 36 or 37,38, respectively, having the cross-sectional configuration chosen forthe slab line center conductor.

' The'articulated construction of the paired arms 35,

and .37, 38 is achieved by a tongue and groove ar 'rangernent, as shownin best detail in the enlarged fragment of FIG. 3. A tongue 40 extendingfrom one of the "arms 37, for exa1nple, mates slidably in a matchinggroove 41in;thefacing end of the coupling element 33, A pivot pin 42extending through the coupling element 33 registers with an appropriateaperture in the tongue 40 andp'ermits a pivotalfmovement of the arm37-relative tothecouplingelement 33. The end surfaces of the variousabut-ting sections are appropriately curved in mating fashion so thatthere is no binding between the arm sections. The relative pivotalmotion which is permitted between various arms is restricted to a planewhich is parallel to the plane of the ground plane conductors Id, 15.The two pairs of articulated arms 35, 36 and 37, 38 are generallysimilar, so that the base leg of the U-shaped central coupling element33 may be maintained parallel to the fixed center conductor 26 despitetransverse movement.

With this arrangement, the central coupling element 33 is movedtransversely with respect to the length of the fixed center conductor26, by a suitable mechanism which may be controlled externally to theframe 12. In the form shown by Way of example, the central couplingelement 33 is fixed to a dielectric insulator 45 which providesmechanical support but which insulates the center conductor sectionsfrom the remainder of the assembly. A rack member 46 is also coupled tothe dielectric insulator 45, and extends between the ground planeconductors i4, 15 in a direction normal to the length of the fixedcenter conductor 26. Teeth in the rack member 46 are engaged by theteeth of a pinion 47, which is rotatably mounted in the top ground planeconductor 14. A control knob 48 mounted outside the frame 12 may be usedto adjust the position of the rack member 46 through rotation of thepinion 47.

Several mechanical alternatives which may be employed have been omittedin orderto simplify and clarify the drawings, inasmuch as these mayassume a wide number of forms and are not essential to the operation ofcouplers in accordance with the invention. They may, however, be brieflydiscussed. In order to position the central coupling element 33 wheredesired, appropriate indicia (not shown) may be included on the topground plane conductor 14 adjacent the controi knob 43. For vernicrcontrol, a step-down gearing or coupling may be used for precise minuteadjustment of the transverse position of the central coupling element33. A setscrew or other lock arrangement may also be employed forretaining the movable elements in position, once a position has beenselected. For firm support of the fixed center conductor 26 or themovable coupling section 28, dielectric elements having a low dielectricconstant may be employed between the various center conductors and theground planes. The central coupling element 33 and the articulated arms35 38 of the movable coupling section 28 are slidably mounted on asupport element 49 shaped to conform to the rack member 46 and aifixedto either or both of the ground plane conductors 14, 15.

Resistive elements 53, 51 or other microwave energy absorbing elementsare mounted between the top and bottom ground plane conductors 14, 15 inthe areas between the fixed center conductor and the differentarticulated arm pairs 35, 3s and 5'7, 3S,respectivcly.

In the operation of the arrangement of FIGS. 13 as a variable coupler,the transverse displacement of the central coupling element 33 from thefixed center conductor 26 in the wave coupling region determines theamount of coupling from a selected input terminal to a selected outputterminal. Taking the first terminal conductor 20 as the source of inputsignals, a very close spacing between the central coupling element 33and the fixed center conductor 26 would at the most cause asubstantially equal split of the waves (3 db of coupling) between thesecond terminal conductor 21 and the third terminal conductor 22. Thedistribution of the electric fields in the coupling region with thecenter conductors 2s and 33' very closely juxtaposed would result insubstantially like electric fields being associated with each of thecenter conductors 26 and 33. In this case, halt the microwave energywould be transmitted directly to the system load 2% at the secondterminal conductor 21, and half would be coupled onto the movablecoupling section 28. The waves coupled onto the central coupling element33 are reversed from the original direc- -device.

pled output signals would appear at the third terminal.

conductor 22 and the device would be a 3 db coupler.

In the majority of instances in which a variable coupler is used, 3 dbof coupling is not sought. The above illustration is merely given todemonstrate the manner in which coupling is achieved.

A significant feature of devices provided in accord ance with thepresent invention is that the transverse displacement of the centralcoupling element 33 from the fixed center conductor 26 determines theamount of coupling with a high degree of linearity. Thus, if the centralcoupling element 33 is moved further away from a selected spatialrelationship to the fixed center conductor 26, the percentage ofcoupling is proportionately reduced. This direct relation betweendisplacement and percentage of coupling which is a logarithmic factor isextremely useful, because percentage of coupling is the parameter whichis controlled in system use.

FIG. 4 shows alternative transverse displacement positions of thecentral coupling element 33 of the movable coupling section 25 relativeto the fixed center conductor 26. Dotted lines between the centerconductors 26, 33 and the top and bottom plates 14, 15 representelectric field distributions in the slab transmission lines. With thepresent arrangement, there is a linear relationship between thepercentage of coupling and the displacement of the center conductors 26,33. -As the movable coupling element 33 is moved transversely away fromthe fixed center conductor 26, the extent of coupling onto the movablecoupling element 33 diminishes. This diminution is at a rateofapproximately 1 db of coupling for 0.010 inch of displacement, in atypical practical For this same practical example, an accuracy of :1 dbis maintained between 5 and 70 db over a two to one frequency range.Thus the percentageof coupling, though a logarithmic variable, isachieved very simply by a linearly varying drive and indicated withcommensurate ease by-equally spaced indicia about the control knob =38or other control element.

7 Another important feature of these devices is their bidirectionaloperation. If input signals. are applied at the second terminalconductor 21, for example, there is direct transmission to the firstterminal conductor 2%, and coupled transmission to'the fourth terminalconductor 23. The operation 'is thereforeprecisely as previouslydescribed except for the changes in the terminals at which the outputsignals appear. Through use of this capability the versatility ofcoupler systems is greatly enhanced. It will also be noted that any orall of thet'erminal conductors Zh-ZS may be shifted in the plane of theground plane conductors by appropriate bends in the center conductors.Thus, two terminal conductors may extend normal to the others toaccommodate particular system requirements. A

In accordance with conventional fixed and variable coupler operations,inwhich the bidirectional properties are employed, waves may bepropagated from any adjacent pair of terminal conductors 2t 22 or 21,'23 in a selected phase relation'so as to 'be combined at a selected Iterminal conductor andcancelled at the other terminal conductor.

A surprising result which is achieved by arrangements I 5 transversedisplacement between the central coupling element 33 and the fixedcenter conductor 26.

The resistive elements 50, 51 or other energy absorbers in the spacesbetween the fixed center conductor 26 and the articulated arms 35-38also contribute materially to the linearity of the device. These cards50, '51 suppress, by dissipation, higher order modes which might be setup along the slab transmission lines.

Devices in accordance with the invention may be used as attenuatorswithout internal modification. The difference between the variablecoupler and the variable attenuator results only from the externalconnections to the device, because with both applications selectedproportions of energy are provided to a utilization device. In theattenuator, energy which is not coupled off is dissipated in a matchedtermination, whereas in the coupler, as shown in FIG. 1, this energy isthe output signal. With an attenuator device, therefore, input signalsmight be applied to the first terminal conductor ,20 while coupled waveoutput signals are derived at the third terminal conductor 22 and thesecond terminal conductor 21 is connected to a matched termination (notshown). The output signal may be. referenced to either the input signalor the percentage of attenuation.. In the coupler, the correspondingcoupled wave will usually be referenced to the input signal;

The variable coupler which also has a variable frequency band is shownin FIGS. 5 and 6. There is a general correspondence between the elementsof FIG. 5

and those of FIGS. 1-3, in that an articulated slab line construction isused to couple between four terminal conductors 20-23. As in the view'ofFIG. 2, the plan view of FIG. 5 is shown with the top ground planeconductor removed. Various sources, utilization devices and matchedloadshave been indicated but not shown in detail. It is assumed thatinput signals are provided to a third terminal conductor 22 which iscoupled by a slab transmission line'section having fixed ends 30, 31,and an intermediate transversely movable coupling section 28 to a fourthterminal conductor 23. The fourth terminal conductor 23 is connected toa system load. This por tion of the arrangement corresponds essentiallyto the like numbered elements in'the arrangement of FIG. 1.

A control knob 48 (FIG. 6) turns a pinion 47 which controls the positionof the transversely movable coupling section 28.

A second movable coupling section 54 couples fixed center conductors 56,57 which are connected directly to the first and second terminalconductors 20, 21. The second movable coupling section 54 includes anL-shaped central coupling element 58 having one leg which is-main tainedparallel to the principal length of the transversely movablecouplingelement 33. A dielectric support member 60 including a toothedrack surface includes extending arms having tongues 61 (FIG. 6)configured to register in grooves in the bottom ground plane conductor1.5, or ,in a support member. The grooves confine the'L- shaped couplingelement 58 to longitudinal movement. That is, the leg of the -L-shapedcoupling element 58 which directly opposes the principal length of thetransversely movable coupling element 28, and which is parallel thereto,moves along only its longitudinal axis. The arm and support mechanismwhich is shown is merely one expedient which may be used.

Pairs of articulated'arms 64, 65 and 66, 57 having the cross-sectionalconfiguration of the slab line center conductor couple each of the fixedcenter conductor segments 5d, 57, respectively, to a different end ofthe L- shaped coupling element 58. The longitudinal position of theL-shaped coupling element 58 is controlled by a pinion 68 whichregisters with the teeth provided in the dielectric support member 60,The pinion 68 is directly connected to an external control knob 69 bywhich frequency range is selected. Resistive elements '74-, or othermicrowave absorbing elements positioned between the top and bottomplates 14, 15 and each extending into the region between the opposedarticulated arm sections dissipate higher ordcr'modes propagated withinthe coupler section. I

The arrangementshown in FIGS- and 6 is adjustable so as to providevariable coupling or attenuation over an extremely wid'e range" of"frequencies. When used as a variable coupler, with input signals coupledinto third terminal conductor 22, output signals appear at the directlyconnect'ed fourth terminal conductor 23 and, in a selected proportion,at the wave coupled to the first terminal conductor 20. A matchedresistive load or other termination coupled to the remaining secondterminal conductor 21, together with the presence of resistive elements74, 75 permits operation with high efficiency. As before, the percentageof coupling is controlled by the transverse displacement between theU-shaped coupling element 33 and the parallel leg of the L-shapedcoupling element 58. For different frequency ranges, however, therelative longitudinal position of these two coupling elements' 33, SSmaybe adjusted so that the parallel lengths of the coupling elements 33, 58are coextensive over a quarter wavelength of the waves at the centraloperating frequency. If it is desired to shift the variable coupler fromthe 2000-4000 rnc. band to the 4000-8000 mc band, for example, thecoextensive length of the two coupling elements 33, 58 need only beadjusted to be a quarter wavelength at 6000 me. instead of at 3000 me.

The percentage of coupling between the two coupling sections is affectedby the relative longitudinal positions of the coupling elements 33, 58,only where frequency sensitivity becomes an important factor because ofimproper setting of the frequency control knob 69. With the frequencycontrol knob 69 initially set to a selected frequency, the length ofexposed coupling loop is a quarter wavelength at the operatingfrequency. Given this proper prior adjustment, a given transversedisplacement, as determined by the control knob 48, provides asubstantially constant percentage of coupling for all frequencies ofinterest. Both fabrication andtad justment-are maten allysimplifiedthereby. u

l. A'variablemicrowave coupler including first and second spaced apartground plane conductors, first, seci ond, third and fourth coaxialterminal conductors positioned about the spaced apart ground planeconductors, a first fixed center conductor positioned between the groundplane conductors and connected to each of the first and second terminalconductors to provide a first slab'transrnission line electricallycoupling the first and second terminal conductors, a second centerconductor positioned between the ground plane conductors and providing aselected coupling length which is substantially one-quarter wavelengthat the operating wavelength of waves propagated along the first slabtransmission line, first and second articulated center conductor means,the first articulated center conductor means being coupled from thethird terminal conductor to the adjacent end of the second centerconductor and the second articulated center conductor means beingcoupled from the fourth terminal conductor to the adjacent end of thesecond center conductor, the first artic ulated center conductor means,the second center conductor and the second articulated center conductormeans providing a second slab transmission line between the third andfourth terminal conductors, means coupled to the second center couductorfor moving the second center conductor transversely relative to thefirst While maintaining the coupling length parallel to the length ofthe first center conductor, thus to provide variable coupling of wavesbetween the first and second wave transmission lines, microwave energyabsorbing means disposed between the first and second slab transmissionlines between the first and second ground plane conductors for thesuppression of higher order modes excited by waves propagated along theslab transmission lines.

2. The invention as set forth in claim 1 above, wherein the means formoving the second center conductor includes dielectric support meanscoupled to the second center conductor, a rack and pinion drive coupledto the dielectric support means, and a control member. containingindicia which denote the relative percentage of coupling between thefirst and second slab transmission lines as determined by the relativetransverse displacement of the first and second center conductors.

3. A variable, adjustable frequency, coupler for microwave applicationsincluding a pair of spaced apart ground plane conductors, a first centerconductor coupling element positioned between the ground planeconductors and forming a portion of a first wave transmission linetherewith, the first coupling element extending along a selected axis,means coupled to the first coupling element for selectively moving thefirst coupling clement parallel to the selected axis, a second centerconductor coupling element positioned between the ground planeconductors parallel to said first coupling element and providing aportion of a second wave transmission line therewith, said secondcoupling element being juxtaposed with respect to said first couplingelement for a predetermined distance the magnitude of which isdetermined by the longitudinal position of the movable said firstcoupling element, means coupled to the second coupling element formoving the second coupling element relative to the first couplingelement in a direction normal to the selected axis, means coupled to thefirst coupling element and completing a wave transmission line betweenfirst and second terminals of said coupler, and means coupled to thesecond coupling element and forming a wave transmission line betweenthird and fourth terminals of said coupler.

References lined in the file of this patent UNITED STATES PATENTS2,531,777 Marshall Nov. 28, 1950 2,615,982 Zaslavsizy Oct. 28, 19522,684,469 Sensiper July 20, 1954 2,794,959 Fox June 4, 1957 FOREIGNPATENTS 749,337 Great Britain May 23, 1956

1. A VARIABLE MICROWAVE COUPLER INCLUDING FIRST AND SECOND SPACED APARTGROUND PLANE CONDUCTORS, FIRST, SECOND, THIRD AND FOURTH COAXIALTERMINAL CONDUCTORS POSITIONED ABOUT THE SPACED APART GROUND PLANECONDUCTORS, A FIRST FIXED CENTER CONDUCTOR POSITIONED BETWEEN THE GROUNDPLANE CONDUCTORS AND CONNECTED TO EACH OF THE FIRST AND SECOND TERMINALCONDUCTORS TO PROVIDE A FIRST SLAB TRANSMISSION LINE ELECTRICALLYCOUPLING THE FIRST AND SECOND TERMINAL CONDUCTORS A SECOND CENTERCONDUCTOR POSITIONED BETWEEN THE GROUND PLANE CONDUCTORS AND PROVIDING ASELECTED COUPLING LENGTH WHICH IS SUBSTANTIALLY ONE-QUARTER WAVELENGTHAT THE OPERATING WAVELENGTH OF WAVES PROPAGATED ALONG THE FIRST SLABTRANSMISSION LINE, FIRST AND ARTICULATED CENTER CONDUCTOR MEANS, THEFIRST ARTICULATED CENTER CONDUCTOR MEANS BEING COUPLED FROM THE THIRDTERMINAL CONDUCTOR TO THE ADJACENT END OF THE SECOND CENTER CONDUCTORAND THE SECOND ARTICULATED CENTER CONDUCTOR MEANS BEING COUPLED FROM THEFOURTH TERMINAL CONDUCTOR TO THE ADJACENT END OF THE SECOND CENTERCONDUCTOR, THE FIRST ARTICULATED CENTER CONDUCTOR MEANS, THE SECONDCENTER CONDUCTOR AND THE SECOND ARTICULATED CENTER CONDUCTOR MEANSPROVIDING A SECOND SLAB TRANSMISSION LINE BETWEEN THE THIRD AND FOURTHTERMINAL CONDUCTORS, MEANS COUPLED TO THE SECOND CENTER CONDUCTOR FORMOVING THE SECOND CENTER CONDUCTOR TRANSVERSELY RELATIVE TO THE FIRSTWHILE MAINTAINING THE COUPLING LENGTH PARALLEL TO THE LENGTH OF THEFIRST CENTER CONDUCTOR, THUS TO PROVIDE VARAIBLE COUPLING OF WAVESBETWEEN THE FIRST AND SECOND WAVE TRANSMISSION LINES, MICROWAVE ENERGYABSORBING MEANS DISPOSED BETWEEN THE FIRST AND SECOND SLAB TRANSMISSIONLINES BETWEEN THE FIRST AND SECOND GROUND PLANE CONDUCTORS FOR THESUPPRESSION OF HIGHER ORDER MODES EXCITED BY WAVES PROPAGATED ALONG THESLAB TRANSMISSION LINES.